1. Field of the Invention
The present invention relates to a constant-current controller, especially to a constant-current controller that generates a stable current for driving a LED light string.
2. Description of the Related Art
With reference to FIG. 2, a control box (60) is connected to a LED light string (62) through a resistor assembly (64) and drives the LED light string (62) according to the pre-determined control program. The LED light string (62) may have lighting units connected in parallel and each lighting unit is further composed of multiple LEDs.
The resistor assembly (64) includes multiple resistors (641) respectively corresponding to the lighting units. Each resistor (641) should be selected properly to have a resistance matching the forward voltage (Vf) of the LEDs. However, the resistor assembly (64) is not integrated in the control box (60), but as a separate device mounted between the control box (60) and the light string (62). When the control box (60) operates, the resistor assembly (64) is not safe because it often heats to a high temperature and eventually burns down. In addition to the safety concern, the separate resistor assembly (64) also causes the aesthetic problems.
With further reference to FIG. 3, the light string (62) may comprise multiple lighting units (621) and each lighting unit (621) is connected with a separate a transistor (T1)(T2). When the transistors (T1)(T2) are turned on, the lighting units (621) are activated by driving currents and emit light. However, such a circuit configuration has to use many wires for connecting the lighting units (621) respectively to a ground.
To overcome the shortcomings, the present invention provides constant-current controller for a LED light string to mitigate or obviate the aforementioned problems.